The present invention relates to a noise filter, and more particularly to a noise filter for three-phase four-wire system of a star connection.
As is well known in the art, noise filters are connected to a portion of the system where noise is generated or where a power source is connected, or at a desired portion of a conductor between a noise generation source and a protected device, so that the generation and the introduction of the noise is prevented and at the same time an erroneous operation of various devices is prevented. To meet high-frequency requirements and restriction against radiated noise, electronic devices and instruments are provided with noise filters as described above. For example, a large-scale electronic computer generally has an input-line construction in which a single-phase switching power source is connected between the voltage conductor carrying each phase and the neutral conductor, i.e., the electric wires or conductors of the star-connection type three-phase four-wire system. Generally, such a switching power source is provided with, as an input portion, a capacitor-input rectifier smoothing circuit.
The noise filter, utilized with the network of such a star-connection type three-phase four-wire system, is generally constructed with a combination of a common coil device and capacitors, as is similar to a single-phase noise filter. The common coil device has four windings. The voltage conductors of R-phase, S-phase and T-phase and neutral conductor are wound on a magnetic core of closed magnetic circuit, so that common-mode noise is canceled.
In a network of star-connection type, three-phase four-wire system, balanced sine-wave current flow to each phase is combined and, as a result, an electric current does not flow through the neutral conductor. Besides, even if the electric current in each phase is unbalanced, an electric current through the neutral conductor is extremely small since the components of each phase substantially cancel each other to provide such a small current.
In the conventional noise filter for three-phase four-wire system, the neutral conductor is not considered to be a conductor as compared with the R, S and T phase conductors, therefore the neutral conductor had a cross sectional area which is equal to, or smaller than a cross sectional area of those other conductors, and it has been considered that such a wire for the neutral conductor did not provide any disadvantages at all. In order to provide a larger currrent-carrying capacity, conductive wires having a larger diameter are used as at a minimum and as necessary. However, it was considered and widely is believed that the neutral conductor, through which only a small current flows, is preferably made of smaller diameter wire in order to simplify the winding operation and to achieve miniaturization of the magnetic core.
The inventors of the present invention prepared a noise filter having a common coil and capacitors wherein four windings constituting the common coil are made of the same conductive wire material, and the thus formed noise filter was inserted to a network of star-connected three-phase four-wire system, and a single-phase switching power source having a capacitor-input type rectifier circuit as an input portion was connected between the voltage connectors of each phase of the output portion and the neutral conductor, and then operated. Although the device was designed such that it had a sufficient margin for carrying the electric current through the voltage conductor of each phase, the temperature rise of the noise filter exceeded the standard of the filter.